Preparation of phthalic anhydride

ABSTRACT

Process for preparing phthalic anhydride by gas-phase oxidation of xylene, naphthalene or mixtures thereof in a shell-and-tube reactor thermostatted by means of a heat transfer medium over three or more different fixed-bed catalysts arranged in zones, wherein the process is carried out so that the maximum temperature in the second catalyst zone in the flow direction is up to 50° C. lower than the maximum temperature in the first catalyst zone and the maximum temperature in the third zone from the gas inlet is from 30 to 100° C. lower than that in the first catalyst zone. The process of the present invention makes it possible to prepare phthalic anhydride in high yields under industrially relevant conditions.

The present invention relates to a process for preparing phthalicanhydride by gas-phase oxidation of xylene, naphthalene or mixturesthereof in a shell-and-tube reactor thermostatted by means of a heattransfer medium over three or more different fixed-bed catalystsarranged in zones.

It is known that phthalic anhydride is prepared industrially bycatalytic gas-phase oxidation of o-xylene or naphthalene inshell-and-tube reactors. The starting material is a mixture of a gascomprising molecular oxygen, for example air, and the o-xylene and/ornaphthalene to be oxidized. The mixture is passed through many tubeswhich are arranged in a reactor (shell-and-tube reactor) and in which abed of at least one catalyst is located. To allow the temperature to beregulated, the tubes are surrounded by a heat transfer medium, forexample a salt melt. Nevertheless, local temperature maxima (hot spots)in which the temperature is higher than in the remainder of the catalystbed can occur in the catalyst bed. These hot spots give rise tosecondary reactions such as total combustion of the starting material orlead to the formation of undesirable by-products which can be separatedfrom the reaction product only with great difficulty, if at all. Inaddition, the catalyst can be irreversibly damaged above a certain hotspot temperature.

The hot spot temperatures are usually in the temperature range from 400to 500° C., in particular in the temperature range from 410 to 460° C.Hot spot temperatures above 500° C. lead to a substantial decrease inthe achievable PA yield and in the catalyst operating life. In contrast,hot spot temperatures which are too low lead to a high content ofunderoxidation products in the phthalic anhydride (in particularphthalide), as a result of which the product quality is significantlyimpaired. The hot spot temperature depends on the xylene loading of theair stream, on the space velocity of the xylene/air mixture over thecatalyst, on the aging state of the catalyst, on the heat transfercharacteristics of the fixed-bed reactor (reactor tube, salt bath) andon the salt bath temperature.

Various measures have been employed for decreasing the intensity ofthese hot spots, and these are described, inter alia, in DE 25 46 268 A,EP 286 448 A, DE 29 48 163 A, EP 163 231 A, DE 41 09 387 A, WO 98/37967and DE 198 23 362 A. In particular, as described in DE 40 13 051 A, useis now made of catalysts of differing activity arranged in zones in thecatalyst bed, with the less active catalyst generally being locatednearer the gas inlet and the more active catalyst being located nearerthe gas outlet. The process is carried out using a two-stage salt bath,with the salt bath temperature of the first reaction zone in the flowdirection of the reaction mixture being kept from 2 to 20° C. higherthan the salt bath temperature of the second reaction zone. The catalystvolume of the first reaction zone is from 30 to 75% by volume and thatof the second reaction zone is from 25 to 70% by volume. The temperatureof the hot spot in the first reaction zone is higher than that in thesecond reaction zone. The difference between the hot spot temperaturesin the modes of operation described in the examples is considerably lessthan 50° C.

DE 28 30 765 A describes a shell-and-tube reactor which is, inter alia,suitable for the preparation of phthalic anhydride using a catalystlocated in two reaction zones. The reaction temperature in the secondreaction zone (i.e. follows from the gas inlet) is higher than that inthe first reaction zone.

DE 29 48 163 A describes a process for preparing phthalic anhydrideusing two different catalysts arranged in zones, with the catalyst ofthe first zone making up from 30 to 70% of the total length of thecatalyst bed and the catalyst of the second zone making up from 70 to30% of the total length of the catalyst bed. This is intended to reducethe temperature of the hot spots. However, it has been found that theyield of phthalic anhydride even at the low o-xylene loadings in thestarting gas mixture used in this publication (not more than 85g/standard m³) leaves something to be desired. A similar disclosure ismade in DE 30 45 624 A.

DE 198 23 262 describes a process for preparing phthalic anhydride usingat least three coated catalysts arranged in superposed zones, with thecatalyst activity increasing from zone to zone from the gas inlet end tothe gas outlet end. In this process, the difference in the hot spottemperature from catalyst to catalyst does not exceed 10° C.

EP-A 1 063 222 describes a process for preparing phthalic anhydridewhich is carried out in one or more fixed-bed reactors. The catalystbeds in the reactors comprise three or more individual catalyst zonesarranged in series in the reactor. After passage through the firstcatalyst zone under the reaction conditions, from 30 to 70% by weight ofthe o-xylene, naphthalene or mixture of the two in the feed have beenreacted. After the second zone, 70% by weight or more have been reacted.

However, the results obtained according to EP-A 1 063 222 are not yetsatisfactory, because the heat of reaction and the conversion of thestarting materials are not distributed uniformly over the reactor, inparticular the catalyst bed, as can be seen from the hot spot profile inFIG. 5 in that publication. Thus, different aging of the catalyst zonesoccurs and this in turn leads to a decrease in the yield after aprolonged period of operation.

It is an object of the present invention to provide a process forpreparing phthalic anhydride which gives high yields of phthalicanhydride even at high o-xylene or naphthalene loads and at high spacevelocities and in which the heat of reaction is distributed moreuniformly over the length of the total catalyst bed, thus contributingto an increased catalyst life.

We have found that this object is achieved if the preparation ofphthalic anhydride is carried out over three or more, preferably fromthree to five, catalysts of differing activity arranged in zones, withthe reaction being controlled so that the hot spot temperature in thesecond catalyst zone from the gas inlet (in the flow direction) is from0 to 50° C. lower than that in the first catalyst zone and the hot spottemperature in the third catalyst zone from the gas inlet (in the flowdirection) is from 30 to 100° C. lower than that in the first catalystzone.

The present invention accordingly provides a process for preparingphthalic anhydride by gas-phase oxidation of xylene, naphthalene ormixtures thereof in a shell-and-tube reactor thermostatted by means of aheat transfer medium over three or more different fixed-bed catalystsarranged in zones, wherein the process is carried out so that themaximum temperature in the second catalyst zone in the flow direction isup to 50° C. lower than the maximum temperature in the first catalystzone and the maximum temperature in the third zone from the gas inlet isfrom 30 to 100° C. lower than that in the first catalyst zone.

The maximum temperature in the second catalyst zone is preferably atleast 10-40° C. lower than the maximum temperature in the first catalystzone. The maximum temperature in the third catalyst zone from the gasinlet (in the flow direction) is preferably from 40 to 80° C. lower thanthe maximum temperature in the first catalyst zone.

Furthermore, the process is carried out so that the hot spot temperaturein the first catalyst zone is less than 470° C. and preferably less than450° C.

The difference in the hot spot temperatures can be brought about invarious ways. For example, it can be achieved by increasing the pressureof the starting gas mixture at the inlet by up to 10% or by lowering theamount of air used for the oxidation by up to 20%. However, thetemperature difference is preferably controlled by means of the bedlength ratio of the three or more catalyst zones or by means of thetemperature of the heat transfer medium (hereinafter, reference willalways be made to the preferred heat transfer medium, namely a saltbath), in particular when the three or more catalyst zones arethermostatted by means of different salt bath circuits. The bed lengthof the first catalyst zone preferably makes up more than 30%, inparticular more than 40%, of the length of the total catalyst bed.

If the salt bath temperature is used for control, an increase in thesalt bath temperature leads to an increase in the hot spot temperaturein the first catalyst zone and to a decrease in the second and eachsubsequent catalyst zone. In general, a slight increase or decrease,e.g. by 1, 2 or 3° C., is sufficient to set the desired hot spottemperature difference. If the three or more catalyst zones arethermostatted by means of different salt bath circuits, the upper saltbath circuit, i.e. the salt bath circuit which thermostats the firstcatalyst zone, is preferably operated at a temperature which is from 0.5to 5° C. higher than that of the lower salt bath circuit. Alternatively,the temperature of the salt bath which thermostats the second catalystzone is reduced by up to 10° C. and the temperature of the salt bathwhich thermostats the third catalyst zone is decreased by a further 10°C.

The operating life of the catalyst is generally from about 4 to 5 years.The activity of the catalyst generally decreases a little over time.This can result in the hot spot temperature difference between the firstand third catalyst zones dropping below the minimum value of 30° C. Itcan then be brought back to a value of 30° C. or above by means of theabove-described increase in the salt bath temperature. The process ispreferably carried out so that the hot spot temperature differences aremaintained for at least the first 50%, in particular at least the first70%, particularly preferably at least the first 90%, of the catalystoperating time and particularly advantageously for essentially theentire catalyst operating time.

The hot spot temperature is determined in a known manner, e.g. byinstallation of a plurality of thermocouples in the reactor.

Oxidic supported catalysts are suitable as catalysts. To preparephthalic anhydride by gas-phase oxidation of o-xylene or naphthalene,use is made of spherical, ring-shaped or shell-shaped supportscomprising a silicate, silicon carbide, porcelain, aluminum oxide,magnesium oxide, tin dioxide, rutile, aluminum silicate, magnesiumsilicate (steatite), zirconium silicate or cerium silicate or mixturesthereof. The catalytically active constituents are generally titaniumdioxide, in particular in the form of its anatase modification, togetherwith vanadium pentoxide. The catalytically active composition mayfurther comprise small amounts of many other oxidic compounds which actas promoters to influence the activity and selectivity of the catalyst,for example by reducing or increasing its activity. Such promoters are,for example, alkali metal oxides, thallium(I) oxide, aluminum oxide,zirconium oxide, iron oxide, nickel oxide, cobalt oxide, manganeseoxide, tin oxide, silver oxide, copper oxide, chromium oxide, molybdenumoxide, tungsten oxide, iridium oxide, tantalum oxide, niobium oxide,arsenic oxide, antimony oxide, cerium oxide and phosphorus pentoxide.The alkali metal oxides act, for example, as promoters which reduce theactivity and increase the selectivity, while oxidic phosphoruscompounds, in particular phosphorus pentoxide, increase the activity ofthe catalyst but reduce its selectivity. Catalysts which can be used aredescribed, for example, in DE 25 10 994, DE 25 47 624, DE 29 14 683, DE25 46 267, DE 40 13 051, WO 98/37965 and WO 98/37967. Coated catalystsin which the catalytically active composition is applied in the form ofa shell or coating to the support (cf., for example, DE 16 42 938 A, DE17 69 998 A and WO 98/37967) have been found to be particularly useful.

The less active catalyst is arranged in the fixed bed so that thereaction gas comes into contact firstly with this catalyst and only thenwith the more active catalyst in the second zone. The reaction gassubsequently comes into contact with the even more active catalystzones. The catalysts of differing activity can be thermostatted to thesame temperature or to different temperatures. In general, a catalystdoped with alkaline metal oxides is used in the first catalyst zonenearest the gas inlet and a catalyst doped with a smaller amount ofalkali metal oxides and/or phosphorus compounds and/or further promotersis used in the second reaction zone. A catalyst doped with still smalleramounts of alkali metal oxides or phosphorus compounds and/or furtherpromoters is used in the third catalyst zone.

Particular preference is given to catalysts having the followingcompositions:

-   -   for the first zone:        -   from 3 to 5% by weight of vanadium pentoxide        -   from 0.1 to 1% by weight of an alkali metal oxide, e.g.            cesium oxide        -   from 94 to 96.9% by weight of titanium dioxide    -   for the second zone:        -   from 4 to 7% by weight of vanadium pentoxide        -   from 0 to 0.5% by weight of an alkali metal oxide, e.g.            cesium oxide        -   from 0.05 to 0.4% by weight of phosphorus pentoxide            (calculated as P)        -   balance to 100% by weight: titanium dioxide    -   for the third zone:        -   from 6 to 9% by weight of vanadium pentoxide        -   from 0 to 0.3% by weight of an alkali metal oxide, e.g.            cesium oxide        -   from 0.05 to 0.4% by weight of phosphorus pentoxide            (calculated as P)        -   if desired, from 1 to 5% by weight of a further promoter, in            particular Sb₂O₃        -   from 85.3 to 93.95% by weight of titanium dioxide

The reaction is generally carried out so that the major part of theo-xylene and/or naphthalene present in the reaction gas is reacted inthe first reaction zone.

For the reaction, the catalysts are introduced in layers (zones) intothe tubes of a shell-and-tube reactor. The reaction gas is passed overthe catalyst bed prepared in this way at temperatures of generally from300 to 450° C., preferably from 320 to 420° C. and particularlypreferably from 340 to 400° C., and a gauge pressure of generally from0.1 to 2.5 bar, preferably from 0.3 to 1.5 bar, and at a space velocityof generally from 750 to 5000 h⁻¹, preferably from 2000 to 5000 h⁻¹. Thereaction gas fed into the catalyst bed (starting gas mixture) isgenerally produced by mixing a gas which comprises molecular oxygen andmay further comprise, in addition to oxygen, appropriate reactionmoderators and/or diluents such as steam, carbon dioxide and/or nitrogenwith the o-xylene or naphthalene to be oxidized. The reaction gasgenerally contains from 1 to 100 mol %, preferably from 2 to 50 mol %and particularly preferably from 10 to 30 mol %, of oxygen. In general,the reaction gas is loaded with from 5 to 140 g/standard m³ of gas,preferably from 60 to 120 g/standard m³ and particularly preferably from80 to 120 g/standard m³, of o-xylene and/or naphthalene.

If desired, a downstream finishing reactor as described, for example, inDE 198 07 018 or DE 20 05 969 A may additionally be provided for thepreparation of phthalic anhydride. The catalyst used here is preferablya catalyst which is even more active than the catalyst of the thirdzone. In particular, this catalyst has the following composition:

-   -   from 6 to 9% by weight of vanadium pentoxide    -   from 1 to 5% by weight of an activity-increasing promoter, in        particular Sb₂O₃    -   from 0.1 to 0.5% by weight of phosphorus pentoxide (calculated        as P)    -   from 85.5 to 92.9% by weight of titanium dioxide.

The process of the present invention has the advantage that phthalicanhydride can be prepared in high yield and with low concentrations ofby-products, in particular phthalide, even at high o-xylene and/ornaphthalene loadings and at high space velocities. Under the conditionsof the process of the present invention, the phthalide concentration isno more than 0.1% by weight, based on PA. The advantages of the processof the present invention are particularly evident when the activity ofthe catalyst system used decreases as a result of aging. Even after along running time, there is only an insignificant increase in the hotspot temperature in the second catalyst zone.

The temperature control method provided according to the presentinvention can also be used in preparation of other products by catalyticgas-phase oxidation, e.g. acrylic acid (from propene), maleic anhydride(from benzene, butene or butadiene), pyromellitic anhydride (fromdurene), benzoic acid (from toluene), isophthalic acid (from m-xylene),terephthalic acid (from p-xylene), acrolein (from propene), methacrylicacid (from isobutene), naphthoquinone (from naphthalene), anthraquinone(from anthracene), acrylonitrile (from propene) and methacrylonitrile(from isobutene).

The following examples illustrate the invention without restricting itsscope.

EXAMPLES

1) Production of the Catalysts I-IV

Catalyst I:

50 kg of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 28.6kg of anatase having a BET surface area of 20 m²/g, 2.19 kg of vanadyloxalate, 0.176 kg of cesium sulfate, 44.1 kg of water and 9.14 kg offormamide until the weight of the applied layer was 10.5% of the totalweight of the finished catalyst (after calcination at 450° C.).

The catalytically active composition applied in this way, i.e. thecatalyst shell, comprised 4.0% by weight of vanadium (calculated asV₂O₅), 0.4% by weight of cesium (calculated as Cs) and 95.6% by weightof titanium dioxide.

Catalyst II:

The procedure for the preparation of catalyst I was repeated using 0.155kg of cesium sulfate, which led to a cesium content of 0.35% by weight(calculated as Cs).

Catalyst III

50 kg of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 28.6kg of anatase having a BET surface area of 20 m²/g, 4.11 kg of vanadyloxalate, 1.03 kg of antimony trioxide, 0.179 kg of ammonium dihydrogenphosphate, 0.045 kg of cesium sulfate, 44.1 kg of water and 9.14 kg offormamide until the weight of the applied layer was 10.5% of the totalweight of the finished catalyst (after calcination at 450° C.).

The catalytically active composition applied in this way, i.e. thecatalyst shell, comprised 0.15% by weight of phosphorus (calculated asP), 7.5% by weight of vanadium (calculated as V₂O₅), 3.2% by weight ofantimony (calculated as Sb₂O₃), 0.1% by weight of cesium (calculated asCs) and 89.05% by weight of titanium dioxide.

Catalyst IV

50 kg of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 28.6kg of anatase having a BET surface area of 11 m²/g, 3.84 kg of vanadyloxalate, 0.80 kg of antimony trioxide, 0.239 kg of ammonium dihydrogenphosphate, 44.1 kg of water and 9.14 kg of formamide until the weight ofthe applied layer was 12.5% of the total weight of the finished catalyst(after calcination at 450° C.).

The catalytically active composition applied in this way, i.e. thecatalyst shell, comprised 0.2% by weight of phosphorus (calculated asP), 7.0% by weight of vanadium (calculated as V₂O₅), 2.5% by weight ofantimony (calculated as Sb₂O₃) and 90.3% by weight of titanium dioxide.

2) Oxidation of o-xylene

2a) Preparation of PA—According to the Present Invention and Comparison

(Setting of the Hot Spot Temperature Difference by Varying the BedLengths)

In a 10 l tube reactor (99 normal tubes and 2 thermocouple-containedtubes), each of the 3.60 m long iron tubes having an internal diameterof 25 mm (thermocouple-containing tubes 29 mm with thermocouple sheath10 mm internal diameter and 30 installed thermocouples (every 10 cm))was charged from the bottom upward with catalyst III (comparison: 1.30m; according to the present invention: 0.70 m), catalyst II (accordingto the present invention: 0.80 m) and subsequently catalyst I (1.70 m(comparison); according to the present invention: 1.50 m). It wasensured by means of pressure matching that the pressure at the inlet ofeach tube was the same. If necessary, a little catalyst I was added orsucked out from the 99 normal tubes; in the case of the twothermocouple-containing tubes, pressure matching was achieved byaddition of inert material in the form of steatite spheres or quartzspheres. The iron tubes were surrounded by a salt melt which was locatedin two separate salt baths to regulate the temperature. The lower saltbath surrounded the tubes from the bottom tube plate to a height of 1.30m, and the upper salt bath surrounded the tubes from the height of 1.30m to the upper tube plate. Air laden with 100 g of 98.5% purity by eighto-xylene per standard m³ of air (after a running-up time of about twomonths) was passed through the tubes from the top downward at a flowrate of 4.0 standard m³/h per tube. After leaving the main reactor, thecrude product gas stream was cooled to 280-290° C. and passed through anadiabatic finishing reactor (internal diameter: 0.45 m, height: 0.99 m)charged with 100 kg of catalyst IV.

The data listed in the following table were obtained in the experiment(running day=day of operation from the first start-up of the catalyst;SBT top=salt bath temperature of the salt bath nearest the reactorinlet; SBT bottom=salt bath temperature of the salt bath nearest thereactor outlet; HS top=hot spot temperature of the catalyst nearest thereactor inlet; HS bottom=hot spot temperature of the catalyst nearestthe reactor outlet; PHD content and xylene content=phthalide content andxylene content, respectively, of the crude product gas downstream of thefinishing reactor, based on phthalic anhydride; PA yield=yield of PA in% by weight based on 100%-pure xylene derived from the analysis of thecrude product gas downstream of the finishing reactor). SBT top Δ T Δ TRunning SBT HS top- top- day bottom HS top HS middle bottom middlebottom PA yield Bed [d] [° C.] [° C.] [° C.] [° C.] [° C.] [° C.] [%]Comparison 100 348/348 434 — 366 — 68 113.1 170/130 150 348/348 434 37557 112.9 200 348/348 421 390 31 112.0 250 348/348 419 394 25 111.3According 100 348/348 430 400 360 30 70 113.3 to the 150 348/348 431 402359 29 71 113.1 present 200 348/348 425 413 361 12 64 112.9 invention250 348/348 421 411 362 10 59 112.7 150/80/702b) Preparation of PA—According to the Present Invention(Temperature Variation and Temperature Structuring)

After the catalyst combination operated in 2a) as comparative experimenthad been run for 250 days, a temperature difference of >40° C. was setby means of temperature structuring (SBT bottom reduced or SBT topincreased) or temperature variation (SBT bottom and top increased). Allother experimental conditions remained unchanged from those inexperiment 2a).

The data listed in the following table were then obtained (runningday=day of operation from the first start-up of the catalyst; SBTtop=salt bath temperature of the salt bath nearest the reactor inlet;SBT bottom=salt bath temperature of the salt bath nearest the reactoroutlet; HS top=hot spot temperature of the catalyst nearest the reactorinlet; HS bottom=hot spot temperature of the catalyst nearest thereactor outlet; PHD content and xylene content=phthalide content andxylene content, respectively, of the crude product gas downstream of thefinishing reactor, based on phthalic anhydride; PA yield=yield of PA in% by weight based on 100%-pure xylene derived from the analysis of thecrude product gas downstream of the finishing reactor). Running SBT top/HS HS Temperature Bed day SBT bottom top bottom difference PA yield170/130 [d] [° C.] [° C.] [° C.] [° C.] [m/m %] Comparison without 250348/348 419 394 25 111.3 temperature structuring according to thepresent 252 349/349 428 387 41 112.3 invention with temperature 254350/350 437 381 56 112.5 increase according to the present 256 349/348429 385 44 112.5 invention with temperature 258 350/348 438 379 58 112.8structuring 260 348/343 419 381 38 112.0 262 348/338 418 370 48 112.9264 348/335 419 365 54 113.1

The results reported under 2a) show that the PA yield correlates withthe hot spot temperature difference, i.e. PA is obtained in high yieldunder industrially relevant operating conditions when the temperaturedifference between the first and second catalyst zones is in the rangefrom 0 to 50° C. and that between the first and third catalyst zones isin the range from 30 to 100° C.

The results reported under 2b) show that when the hot spot temperaturedifference is too low, the hot spot temperature difference in thedual-structure bed can be increased again either by simultaneouslyincreasing the salt bath temperature top and bottom to a small extent orby reducing the temperature of the lower salt bath while maintaining thesame temperature in the upper salt bath.

1. A process for preparing phthalic anhydride by gas-phase oxidation ofxylene, naphthalene or mixtures thereof in a shell-and-tube reactorthermostated by means of a heat transfer medium over three or moredifferent fixed-bed catalysts arranged in zones, wherein the process iscarried out so that the maximum temperature in the second catalyst zonein the flow direction is from 10 to 40° C. lower than the maximumtemperature in the first catalyst zone and the maximum temperature inthe third zone from the gas inlet is from 30 to 100° C. lower than thatin the first catalyst zone.
 2. A process as claimed in claim 1, whereinthe maximum temperature in the third catalyst zone is from 40 to 80° C.lower than in the first catalyst zone.
 3. A process as claimed in claim1, wherein the temperature difference between the maximum temperature inthe first, second and third catalyst zones is controlled by means of thebed length ratio of the catalyst zones.
 4. A process as claimed in claim3, wherein the bed length of the first catalyst zone is more than 30% ofthe length of the total catalyst bed.
 5. A process as claimed in claim3, wherein the bed length of the first catalyst zone is more than 40% ofthe length of the total catalyst bed.
 6. A process as claimed in claim1, wherein the temperature difference between the maximum temperature inthe first catalyst zone and in the second catalyst zone is controlledvia the temperature of the heat transfer medium.
 7. A process as claimedin claim 1, wherein the maximum temperature in the first catalyst zoneis less than 470° C.
 8. A process as claimed in claim 1, wherein a gasphase having a loading of from 80 to 140 g of o-xylene and/ornaphthalene per standard m³ of gas phase is used.
 9. A process asclaimed in claim 1, wherein the temperature of the heat transfer mediumis ≦360° C.
 10. A process as claimed in claim 1 wherein the spacevelocity of the gas mixture is ≧2000 h⁻¹.